clocksource: sh_cmt: Rename mapbase/mapbase_str to mapbase_ch/mapbase

The mapbase variable points to the mapped base address of the channel,
rename it to mapbase_sh. mapbase_str points to the mapped base address
of the CMT device, rename it to mapbase.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>